WO1996022864A1

WO1996022864A1 - Laser-machineable articles

Info

Publication number: WO1996022864A1
Application number: PCT/US1996/001455
Authority: WO
Inventors: John Swanson
Original assignee: Pi Medical Corporation
Priority date: 1995-01-25
Filing date: 1996-01-24
Publication date: 1996-08-01

Abstract

A method of shaping an article (10) made from dimethylphenyl-substituted siloxane, the key step of which is UV laser ablation of the article (10) by a UV laser beam (13) delivered from a focusing lens (12) to form microvoids (25) in the surface of the article (10).

Description

LASER-MACHINEABLE ARTICLES

Technical Field

The present invention has broad application in the field of the micromachining of articles of manufac¬ ture such as in the fabrication of circuit boards, optical wave guides, diffraction grating, industrial sensors, and microporous membranes, and has particular application in the field of fabrication of biomedical devices, and more specifically of prostheses, implants and drug delivery systems. It further has application in the fabrication of prostheses and implants that include electrically conductive material, such as electrodes for placement within living beings to stimulate tissue elec- trically and to detect electrical activity in living tissue. Specific examples of such devices include cochlear implants, nerve cuff electrodes, subdural strip electrodes such as epileptogenic mapping sensors, cortical electrical sensors, catheters, catheter sensors, cardiac leads for pacemakers and for defibrillation, and electrode arrays of various configurations.

Background Art

The use of polymeric silicone materials for medical prostheses and implants is well known. The use of microporous polymeric reservoirs for the delivery of drugs is also known. Similarly, the use of osmotically driven capsules formed of polymeric materials and having ports through the capsule wall for the delivery of drugs is known.

The use of certain biologically compatible dielectric materials as substrates or coatings for neural prostheses that incorporate electrical stimulators or sensors is also known. The class of polymers commonly known as parylenes is known to have the required biological compatibility and electrical insulation qualities and can be applied successfully to electrode surfaces. Similarly, silicone resins are known to possess dielectric properties and to be useful in electrically conductive bio edical devices. See, for example, U.S. Patent Nos. 4,602,624, 4,735,208, 4,903,702 and 5,037,497.

Fabrication of such prostheses and implants tends to be complex in the sense that it requires a large number of steps that generally include coating, molding, potting or laminating the polymeric material over wires or electrodes. Three exemplary complex fabrication techniques are disclosed in U.S. Patent Nos. 4,602,624, 4,903,702 and 5,037,497.

U.S. Patent No. 4,602,624 discloses the fabrication of a neural cuff by clamping the ends of a substrate sheet of elastomeric material across a first steel plate, stretching it, cutting out windows of a predetermined size, placing conductive foil over the windows, coating exposed metal with polymeric material to insulate the same, overcoating the entire substrate sheet and metal with uncured silicone elastomer and placing thereover another silicone elastomeric sheet, clamping the entire assembly between the first steel plate and a second steel plate, curing the uncured silicone elastomer and then trimming the so-laminated assembly to the desired dimensions.

U.S. Patent No. 4,903,702 discloses the fabrication of an epileptogenic mapping sensor by cutting two strips of flexible silicone dielectric, one with windows cut therein, placing insulated leads and associ- ated contacts between the strips and in alignment with the windows, placing a radioopaque dielectric ring between one contact and the bottom strip, and then laminating the assembly by heat.

U.S. Patent No. 5,037,497 discloses the fabrication of an array of recessed radially oriented bipolar electrodes suitable for use as an auditory prosthesis by a complex seven-step injection molding process that requires the formation of a multiplicity of dielectric annuli and the application of a vacuum to hold the annuli and electrodes in place during molding.

In the fabrication of electrically conductive bio edical devices the selective removal of dielectric material must be done as precisely as possible. Various techniques for the removal of dielectric material have been used in the past, but they have been largely unsuc¬ cessful. Such techniques have included mechanical removal, AC electric corona arcing, direct heating, masking and plasma etching. These methods all have drawbacks however, either because they fail to leave a cleanly and accurately exposed electrode or sensor surface, or because the remaining adjacent insulating coating does not adhere sufficiently tightly to the microelectrode adjacent the exposed surfaces.

Use of long wave-length lasers to pierce the vinyl lacquer insulation of tungsten microelectrodes was described by M. J. Mela in 1965 in an article entitled "Microperforation with Laser Beam in the Preparation of Microelectrodes," published in IEEE Transactions on Biomedical Engineering, Vol. BME-13, No. 2, pp. 70-76. Mela disclosed use of a non-UN ruby red laser however, which is not capable of cleanly removing dielectric coatings such as parylene and silicone resins.

There is therefore a need in the art for a simple method of shaping and fabricating articles from biocompatible dielectric material that permits selective and precise removal of the material, that permits the adherence of remaining material to the substrate or elec¬ trode, that permits three-dimensional shaping, and that is compatible with fabrication by all known methods, including molding, coating, potting and laminating. These needs and others are met by the process of the present invention, which is summarized and described in detail below. Disclosure of the Invention

The essence of the present invention lies in the discovery that a certain class of dielectric material is ablatable by laser UN irradiation, and so is cleanly removable and formable into predetermined shapes, textures and patterns thereby.

According to the invention, an appropriately focused laser beam of UV light is directed onto a substrate of dimethylphenyl-substituted siloxane ("DMPS") to ablate the siloxane material to selectively remove portions thereof or to form the substrate into a prede¬ termined shape or to render it microporous or to define an active electrode site on a portion of an electrode conductor body.

Brief Description of the Drawings

FIGS. 1-3 are schematics of various substrates comprising DMPS being laser-ablated to form the same into predetermined shapes and textures. FIGS. 4-5 are schematics of an exemplary biologically implantable, multiple-conductor microelectrode.

FIGS. 6a and 6b are schematics of an exemplary biologically implantable epileptogenic mapping sensor.

Best Modes for Carrying Out the Invention

According to the present invention it has been discovered that the dielectric material comprising dimethylphenyl-substituted polysiloxanes may be cleanly ablated by UN laser beams, making such material ideally suited for "machining" on a microscopic scale, which in turn allows for the simple fabrication of biomedical and drug delivery devices, including, but not limited to, protheses, cochlear implants, intraocular implants, electrically conductive muscle stimulators and sensors, osmotic drug delivery capsules and microporous membranes. More specifically, functional group-terminated homopolymers, copolymers and terpolymers of DMPS having a wide degree of phenyl substitution comprise a suitable laser-ablatable material that may readily be removed so as to form a substrate of the material into a predetermined morphology or shape.

The siloxane material is represented by the structure

wherein Me is methyl; Ph is phenyl;

R., and R₂ are selected from hydrogen, halogen, methyl, phenyl, vinyl, acrylyl, methacrylyl, alkynyl and epoxyl;

Z is a functional group selected from vinyl, amino, methyleneamino, halogen, methylenehalo, methoxy, ethoxy, methylethoxy, acetoxy, methylacetoxy, acryloxy, methacryloxy, ercapto, -COX, -S0-.X, -NCO, -NRCOX, -RN=NR, -POXR', and -R00R; and X is halogen; R is -H or R' or R";

R' is alkyl, alkenyl, alkoxy or aryl containing from 1 to 18 carbon atoms;

R" is alkylene, alkenylene, alkoxylene or arylene containing from 1 to 18 carbon atoms; and n is an integer from 400 to 1300.

Such dimethylphenyl-substituted siloxanes are photopolymerizable, with polymerization being initiated by free radical initiators. Exemplary classes of free radical initiators include azos, peroxides, epoxides, certain ketones, aryliodonium salts and aryliosulfonium salts. Specific examples of such initiators include azo- is-isobutyronitrile, dibenzoylperoxide, epichlorohydrin, 1-hydroxycyclohexyl phenyl ketone, 2-hydroxy-2-methyl-1-phenyl-propan-1-one, benzophenone and ,2-diethoxyacetophenone.

The degree of phenyl substitution of such DMPS material is from 5 to 50 mol%, preferably from 10 to 30 mol%, and most preferably about 15 mol%. The DMPS may be modified with silica or other compatible filler, vary¬ ing in amounts from 0 to 35 wt%. The molecular weight may vary from 15,000 ± 5000 to 50,000 ± 20,000. The ratio of terminal functional groups to SiO units may vary from 1:20 to 1:650, and is preferably in the range of 1:200 to 1:550.

Referring now to the drawings, wherein the same numerals denote the same elements, there is shown in FIG. la a cross-sectional view of a DMPS substrate 10 situated under a focusing lens 12 for a UN laser beam. FIG. lb shows the substrate 10 with a channel 15 formed therein by ablation from UV laser beam 13. FIG. lc shows the substrate 10 having a series of microvoids 25 formed therein by UV laser ablation, so as to effectively form a textured surface. FIG. Id shows the substrate 10 having microholes 35 formed therethrough by UV laser ablation, so as to render the substrate microporous. FIG. le is substantially the same as FIG. lc except that the substrate 10 has been coated with a non-UV-absorbing material 11, such as Teflon*, which is transparent to the wavelength of the laser beam.

FIG. 2 is a cross-sectional schematic of a conductive wire 40 coated with a different laser- ablatable coating 30 of parylene, which in turn is surrounded by a cylindrical DMPS substrate 20, the wire being exposed by laser ablation that forms an opening 45 through both coating 30 and substrate 20.

FIG. 3 is a cross-sectional view of a microelectrode comprising a DMPS substrate 22 surrounding an electrically conductive body 42 that is exposed by laser ablation. The DMPS material absorbs laser radiation generally in the UV range. More specifically, and for example, a frequency-quadrupled YAG (FQY) laser operated in the fundamental transverse electromagnetic (TEM^,) mode is suitable to ablate portions of the coating.

Typically, this laser is Q-switched at around 1-20 KHz, producing a 40-50 ns full-width half maximum (F HM) pulse which is focused by a focusing lens to a spot about <25 μm, producing a fluence of approximately 1-50 joules/cm², at an average power of 5-500 milliwatts. Such a laser has a 266 nanometer wavelength which is in the UN range.

Such a highly focused laser beam in the ultraviolet frequency band is readily absorbed by the DMPS dielectric coating material described herein, and is also absorbed by the surfaces of metals such as platinum or iridium, typically used as the electrically conductive metals in biomedical prostheses, with the result that the DMPS dielectric material is both vaporized and photo- ablated, removing it cleanly from the surfaces of the conductive metal. The mechanism by which such ablation of the DMPS dielectric material occurs is not definitely known, but it is believed to be by a combination of conversion of the laser light energy to heat in the DMPS material and the underlying metal, which appears to evaporate the DMPS material without leaving an ash, and further by chemical dissociation of the polymeric DMPS material induced by the UN light energy. Because the underlying surfaces of the electrically conductive metal are heated very quickly to temperatures apparently exceeding 2000°C, which vaporizes DMPS, the surface of the metal is thoroughly cleaned so that it becomes a relatively low resistance contact surface for the conduc¬ tion of electrical current. A final effect of the UV laser ablation is to leave a shallow surface layer of fused DMPS material surrounding the cleaned metal surface, which closely adheres thereto. The FQY laser beam spot can be moved by computer software control to scan the DMPS dielectric material to selectively and cleanly remove it, thereby shaping the DMPS material into virtually any prede- termined shape, texture or even porosity in the event microscopic holes are produced. Scanning control may be provided, for example, by equipment designed to control lasers for use in manufacture of integrated circuit products, such as is available from Electro Scientific Industries, Inc. , of Beaverton, Oregon. Preferably, the UN laser is utilized together with exhaust and positive gas pressure systems to keep debris away from the focus¬ ing lens and the area where dielectric material is being ablated. Operation of the laser at powers of 5-500 milliwatts provides an effective range of etch depths of approximately 5-150 microns in DMDPS.

FIGS. 4-5 show various aspects of an exemplary biologically implantable multiconductor microelectrode 70 capable of being fabricated according to the present invention. The microelectrode 70 includes an electric¬ ally conductive core 72 and fine wires 74 extending from a helically stranded multiconductor cable, each wire being insulated by coatings 76 of flexible dielectric material. The fine wires 74 are wrapped around the conductive core 72 in a helical serving in which the individual fine wires 74 lie alongside one another without overlapping. Each of the fine wires 74 has an individual thin coating 76 of dielectric material, and a further coating 78 of DMPS dielectric material adhesively attaches the fine wires 74 to one another and to the core 72, forming a monolithic, elongate multiconductor microelectrode body.

Using the UV laser beam as previously described, an active electrode site 86 is provided on each of the fine wires 74 by ablating the DMPS dielectric material of the coatings 76 and 78 from the wire over a micro-area, forming a circular opening 88 through the DMPS material 78 and other dielectric material 76 insulating each of the fine wires 74.

Depending upon the intended use of the electrode, the openings 88 of the electrode sites 86 may be left as depressions relative to the outer surface of the outer coating 78 of DMPS dielectric material, or deposits of metal 90 having desired conductivity and resistance to electrochemical corrosion may be provided by electrophoretic deposition, the deposits 90 being attached to and electrically interconnected to the surface of fine wires 74, either partly filling the openings 88, as shown in FIG. 5b, filling the openings flush with the outer surface of the DMPS dielectric material, as shown with deposit 92 in FIG. 5c, or forming small bumps standing proud above the outer surface of the DMPS dielectric material, as shown with deposit 94 shown in FIG. 5d.

For use in situations where a surface contact with a nerve is desired, such as in the case of an epileptogenic mapping sensor, a generally planar array of active electrode sites 100 of a biologically implantable multiconductor microelectrode 120, shown in plan and cross-section in FIGS. 6a and 6b, respectively, may be fabricated according to the present invention and used in connection with a biologically implantable cable 130 similar to that disclosed in commonly owned U.S. Patent No. 5,201,903, the disclosure of which is hereby incorpo¬ rated herein by reference. A series of fine wires 103 extends to each electrode site 100 of the microelectrode 120, which has a generally planar body comprising a lamination of two strips of DMPS material 110a and 110b and electrically conductive foil 105, best seen in FIG. 6b. Active electrode sites 100 are provided by ablating openings through the DMPS dielectric material of the body 110 with the use of a UV laser as previously described, to provide a clean electrically conductive contact. Example To demonstrate the peculiarly attractive laser-ablatability of the DMDPS material, 100 mμ-thick coatings of vinyl-terminated poly(dimethylphenylsiloxane) commercial available from McGhan Nusil Corporation of Carpenteria, California as Nusil R-2655 having a degree of phenyl substitution of 5 mol% were made on a 304 stainless steel substrate. Coatings of the same thick¬ ness of vinyl-terminated dimethylsiloxane having no phenyl substitution were made on the same substrate immediately adjacent the phenyl-substituted siloxane coatings. Sets of such coatings were irradiated with 3 passes each of a 266 nanometer laser UN beam of the type described above, varying in power from 10 to 50 mW and in intensity from about 1 to about 5 joules/cm²/pass. In all cases, the non-phenyl-substituted material was observed to have merely been charred or scored in an irregular, jagged and discontinuous pattern, while the phenyl- substituted siloxane was observed to have sharply defined clean cuts through the material so as to expose the shiny metal of the stainless steel substrate.

The terms and expressions employed in the foregoing specification are used therein as terms of description and not of limitation, and there is no intention, in the use of such terms and expressions, of excluding equivalents of the features shown and described or portions thereof, it being recognized that the scope of the invention is defined and limited only by the claims which follow.

Claims

WE CLAIM:

1. A method of shaping an article of dimethyl phenyl-substituted siloxane comprising ablating said article by ultraviolet laser radiation.

2. A method of fabricating a biomedical device comprising the steps of:

(a) providing a substrate of a dimethyl phenyl-substituted siloxane;

(b) ablating said substrate by ultraviolet laser ultraviolet radiation.

3. The method of claim 1 or 2 wherein the degree of substitution of phenyl groups in said siloxane is from 5 to 95 mol%.

4. The method of claim 1 or 2 wherein the degree of substitution of phenyl groups in said siloxane is from 10 to 30 mol%.

5. The method of claim 1 or 2 wherein said siloxane includes silica filler.

6. The method of claim 1 or 2 wherein said ablating is conducted by a laser beam having a fluence of about 1 to about 50 joules/cm²/pulse.

7. The method of claim 1 wherein said article has at least one hole perforated therein by said ablating.

8. The method of claim 1 wherein said article is rendered microporous by said ablating.

9. The method of claim 1 wherein the surface of said article is textured by said ablating.

10. The method of claim 2 wherein said substrate includes electrically conductive metal.

11. The method of claim 10 wherein said metal is embedded in said substrate.

12. The method of claim 10 or 11 wherein said metal is exposed by step (b) .

13. The method of claim 10 wherein said substrate is an electrode comprising an electrically conductive probe coated with said siloxane.

14. The method of claim 13 wherein said electrode is a microelectrode.

15. The method of claim 13 wherein said electrode is a stimulating electrode.

16. The method of claim 13 wherein said electrode is a sensing electrode.

17. The method of claim 13 wherein said probe is coated with a coating of said siloxane having a thickness of 5 to 150 microns.

18. The method of claim 13 wherein said probe has an array of electrodes.

19. The method of claim 18 wherein said array of electrodes is spatially distributed on said probe.

20. The method of claim 19 wherein each electrode in said array of electrodes has an associated electrical conductor.